Adsorbents adsorption/desorption kinetics

In this review we put less emphasis on the physics and chemistry of surface processes, for which we refer the reader to recent reviews of adsorption-desorption kinetics which are contained in two books [2,3] with chapters by the present authors where further references to earher work can be found. These articles also discuss relevant experimental techniques employed in the study of surface kinetics and appropriate methods of data analysis. Here we give details of how to set up models under basically two different kinetic conditions, namely (/) when the adsorbate remains in quasi-equihbrium during the relevant processes, in which case nonequilibrium thermodynamics provides the needed framework, and (n) when surface nonequilibrium effects become important and nonequilibrium statistical mechanics becomes the appropriate vehicle. For both approaches we will restrict ourselves to systems for which appropriate lattice gas models can be set up. Further associated theoretical reviews are by Lombardo and Bell [4] with emphasis on Monte Carlo simulations, by Brivio and Grimley [5] on dynamics, and by Persson [6] on the lattice gas model. 

Adsorption/desorption kinetics the time of the adsorption-regeneration cycle greatly depends on the kinetics of the C02 adsorption-desorption profile, which is measured in breakthrough experiments. Sorbents that adsorb and desorb C02 in a shorter time are preferred as these reduce the cycle time as well as the amount of sorbent required, and ultimately the cost of C02 separation. 

The adsorption/desorption kinetics of Zn on soils with the removal of organic matter and Fe oxides were studied by Kingery et al. (1999) using traditional batch experiments on the same soil used by Hinz and Selim (1999). Adsorption of Zn on the three soils was kinetically characterized by three kinetic rates a very fast initial retention process within 30 min, an intermediate retention process from 30 min to 8 hours, followed by the very slow process (Fig. 5.5). About 50% of Zn was adsorbed on the three soils within the initial first 30 min. Initially, Zn desorption increased with time during the first 20 minutes from both the untreated soils and the soils without organic matter and Fe oxides. However, after 20 minutes of adsorption, net Zn adsorption from the whole soil and the soil with the... 

Analytes may accumulate in the sorption phase either by adsorption onto the surface of solid sorbent materials or by absorption in absorbent liquids or polymers that behave like subcooled liquids.The advantage of solid adsorbents is the potential to select materials with a high affinity and selectivity for target analytes. However, the sorption capacity of adsorbents is usually limited, and the description of adsorption/desorption kinetics of analytes to adsorbents is complex. Typically, the adsorbent materials used in passive samplers are similar to those used in solid-phase extraction techniques. 

Long ago, Langmuir suggested that the rate of deposition of particles on a surface is proportional to the density of particles in the vicinity of the surface and to the available area on the surface [1], However, the calculation of the available area is still an open problem. In a first approximation, one can assume that the available area is the total area of the surface minus the area already occupied by the adsorbed particles [1]. A better approximation can be obtained if the adsorbed particles, assumed to have the shape of a disk, are in thermal equilibrium on the surface, either because of surface diffusion and/or of adsorption/desorption kinetics. In this case, one can use one of the empirical equations available for the compressibility of a 2D gas of hard disks, calculate the chemical potential in excess to that of an ideal gas [2] and then use the Widom relation between the area available to one particle and its excess chemical potential on the surface (the particle insertion method) [3], The method is accurate at low densities of adsorbed particles, where the equations of state are accurate, but, in general, poor at high concentrations. The equations of state for hard disks are based on the virial expansion and only the first few coefficients of this... 

Abstract Investigations of alternate adsorption regularities of cationic polyelectrolytes a) copolymer of styrene and dimethylaminopropyl-maleimide (CSDAPM) and b) poly(diallyldimethylammonium chloride) (PDADMAC) and anionic surfactant - sodium dodecyl sulfate (SDS) on fused quartz surface were carried out by capillary electrokinetic method. The adsorption/desorption kinetics, structure and properties of adsorbed layers for both polyelectrolytes and also for the second adsorbed layer were studied in dependence on different conditions molecular weight of polyelectrolyte, surfactant and polyelectrolyte concentration, the solution flow rate through the capillary during the adsorption, adsorbed layer formation... 

Fig. 38. (a, b) SFG spectra of CO adsorbed on Rh(l 11) at 300K at pressures between 10 and 1000 mbar. (c) Analysis of the on-top CO intensity (surface density), resonance position, and CO coverage as a function of the CO pressure. The open symbols indicate the pressure range of irreversible CO adsorption. The equilibrium CO surface coverage in (c) was calculated from adsorption/desorption kinetics adapted from Pery et al. (314). Copyright (2002) The Combustion Institute. 

The breakthrough came at the beginning of the eighties of the 20" century with the new theoretical approach called the Statistical Rate Theory (SRT), linking the rate of adsorption/desorption kinetics to the chemical potentials of the molecules in the bulk and the adsorbed phases. [4]... 

Recently the new SRT approach has been generalized further to take into account the energetic heterogeneity of the actual adsorption systems and the possible role of the interactions between the adsorbed molecules. [10-20] Most recently, the authors have shown, that the SRT approach can be successfully applied to describe the multi-site-occupancy adsorption of molecules which do not dissociate after being adsorbed. [14] Compact simple analytical expressions were developed, and next used successfully to correlate experimental data for adsorption/desorption kinetics in various gas/solid systems. The purpose of this presentation is to show, that the new SRT approach can be also applied to represent the kinetics of dissociative gas adsorption on solids. That kinetics is of a crucial importance in a variety of catalytic reactions occurring on solid surfaces. 

For the investigation of adsorption/desorption kinetics and surface diffusion rates, SECM is employed to locally perturb adsorption/desorption equilibria and measure the resulting flux of adsorbate from a surface. In this application, the technique is termed scanning electrochemical induced desorption (SECMID) (1), but historically this represents the first use of SECM in an equilibrium perturbation mode of operation. Later developments of this mode are highlighted towards the end of Sec. II.C. The principles of SECMID are illustrated schematically in Figure 2, with specific reference to proton adsorption/desorption at a metal oxide/aqueous interface, although the technique should be applicable to any solid/liquid interface, provided that the adsorbate of interest can be detected amperometrically. 

When the adsorption/desorption kinetics are slow compared to the rate of diffusional mass transfer through the tip/substrate gap, the system responds sluggishly to depletion of the solution component of the adsorbate close to the interface and the current-time characteristics tend towards those predicted for an inert substrate. As the kinetics increase, the response to the perturbation in the interfacial equilibrium is more rapid and, at short to moderate times, the additional source of protons from the induced-desorption process increases the current compared to that for an inert surface. This occurs up to a limit where the interfacial kinetics are sufficiently fast that the adsorption/desorption process is essentially always at equilibrium on the time scale of SECM measurements. For the case shown in Figure 3 this is effectively reached when Ka = Kd= 1000. In the absence of surface diffusion, at times sufficiently long for the system to attain a true steady state, the UME currents for all kinetic cases approach the value for an inert substrate. In this situation, the adsorption/desorption process reaches a new equilibrium (governed by the local solution concentration of the target species adjacent to the substrate/solution interface) and the tip current depends only on the rate of (hindered) diffusion through solution. 

Three main types of mass transfer resistances are recognized film diffusion (which occnrs at the external surface of the adsorbent), intraparticle diffusion (which occnrs within the pores or amorphous structure of the adsorbent), and adsorption/desorption kinetics (which occnrs at the internal surface of the adsorbent). 

Adsorption from liquids onto solid adsorbents is widely utilized in liquid chromatography (HPLC, see Chapter 11,4), described in detail in physical chemistry and instrumental methods textbooks, e.g. in [16]. The separation by HPLC is based on adsorption-desorption kinetics, i.e. on how long various dissolved compounds remain present in the adsorbed state. The average time, ta, during which molecules are present in an adsorption layer is... 

The result is that, initially, net adsorption takes place, reducing the concentration of the effluent stream. After a while, when the temperature becomes high enough, desorption begins to dominate. This raises the concentration of adsorbate in the effluent stream until the adsorbate content of the solid is exhausted. Depending on the equilibrium established by the adsorption/desorption kinetics, this point is approached more or less rapidly at some more or less elevated temperature. The effluent concentration curve therefore looks as shown in Figure 5.10. For clarity, Figure 5.10 shows just two ramp-... 

In comparing the spectral features of all three optical experiments dealing with the Cu(llO) surface it is obvious that the resonance at 2.1 eV is present and dominant in all data sets. The sensitivity of electronic surface states (of clean Cu(llO)) to adsorbed oxygen (and probably other adsorbates) causes in this case the optical techniques to be quite sensitive to adsorbates, enabling their use to monitor the kinetics of adsorption/desorption kinetics, for example. Since electronic surface states are quite common for a number of semiconductor surfaces, it is understandable that optical response investigations are sensitive to adsorbates especially on these surfaces. Hence they are frequently employed to study kinetic phenomena involving adsorption or thin film growth. 

In the following sections we show that almost all these difficulties related to the use of the ART approaeh disappear when we use one of the new approaches relating the rate of adsorption desorption kinetics to the chemical potential of adsorbed molecules. 

At the beginning of the 1980s a new family of approaches to adsorption/desorption kinetics appeared. A common fundamental feature of these approaches is that they relate the rate of adsorption/desorption kinetics to the chemical potential of the adsorbed molecules, Although all these approaches appeared at approximately... 

Furthermore, modeling of the esterification reaction was attempted in the presence of silica nanoparticles during the formation of aliphatic polyester nanocomposites. From the experimental data, it was found that on increasing the Si02 content in esterification, the rate of water production decreases [47]. In addition, it was clear that the total quantity of water released does not depend on the nanoparticle concentration. This suggests that the existence of the particles does not influence the esterification reaction itself. Their main effect is to adsorb the produced water before it evaporates, altering in this way the water evaporation curve. The simplest model for this phenomenon is to assume very fast water adsorption/desorption kinetics on the Si02 particles. In this case, the evaporation kinetics must be explicitly taken into account because it is no more very fast compared to the other phenomena that occur. 

A simple approach to capture the ammonia adsorption/desorption kinetics is the single-site approach, where NH3 is assumed to adsorb on a global single-surface site. A nonactivated ammonia adsorption process is considered while a Temkin-type coverage dependence of the activation energy is assumed for the desorption process [24]. The reaction rate expression of adsorption is given in Eq. (3.21) ... 

It is clear from these experiments that two stable Ga coverages exist, which condense at low temperatures into a (1 x 2)/(4 X 2) reconstruction at bUayer coverage and a (4 x 4) reconstruction at bilayer coverage. On surfaces with noninteger amounts of adsorbed Ga, both reconstructions may coexist (presumably in the form of domains), although we have no experimental evidence for such a coexistence. Furthermore, the recovery behavior of the surface at elevated temperatures provides the opportunity to study the adsorption/desorption kinetics of Ga in real time. 

Eqs. (1,4,5) show that to determine the equilibrium properties of an adsorbate and also the adsorption-desorption and dissociation kinetics under quasi-equilibrium conditions we need to calculate the chemical potential as a function of coverage and temperature. We illustrate this by considering a single-component adsorbate. The case of dissociative equilibrium with both atoms and molecules present on the surface has recently been given elsewhere [11]. 

Additional applications of the transfer matrix method to adsorption and desorption kinetics deal with other molecules on low index metal surfaces [40-46], multilayers [47-49], multi-site stepped surfaces [50], and co-adsorbates [51-55]. A similar approach has been used to study electrochemical systems. 

Analysis of the dynamics of SCR catalysts is also very important. It has been shown that surface heterogeneity must be considered to describe transient kinetics of NH3 adsorption-desorption and that the rate of NO conversion does not depend on the ammonia surface coverage above a critical value [79], There is probably a reservoir of adsorbed species which may migrate during the catalytic reaction to the active vanadium sites. It was also noted in these studies that ammonia desorption is a much slower process than ammonia adsorption, the rate of the latter being comparable to that of the surface reaction. In the S02 oxidation on the same catalysts, it was also noted in transient experiments [80] that the build up/depletion of sulphates at the catalyst surface is rate controlling in S02 oxidation. 

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